When the content of a page is wide (such as a wide table or image), resizing the preview page left or right causes the far right side of the preview pane to be pushed out of sight to the right of the viewport.
It only happens on Vector-2022: https://en.wikipedia.beta.wmflabs.org/w/index.php?title=Phonos&action=edit&useskin=vector-2022
And works correctly on Vector: https://en.wikipedia.beta.wmflabs.org/w/index.php?title=Phonos&action=edit&useskin=vector (and other skins).